Advance publicity for The Ghost has been a publisher's dream of the best, gossipy kind. Robert Harris's narrative centres on Adam Lang, former British premier, holed up in a swanky holiday home in Martha's Vineyard with his spiky-haired wife Ruth. Betrayed by a former close Cabinet colleague, Lang is awaiting indictment on a charge of war crimes after a disastrous Middle East conflict, when he hires a cynical ghost-writer to pen his memoirs.

And if those bare details weren't sufficient provocation to those still picking over the carcass of the Blair years, the charismatic politician is clearly having an affair with his sexy blonde aide. As one journalist wrote brutally last week, "It does not take a huge leap of imagination to see this as a fictionalised attempt by Harris to stab Tony and Cherie Blair firmly in the front."

The speculation has added force because Harris is one of the few writers who can claim insider knowledge of Blair and his entourage. An eminent political journalist, and biographer of former Labour leader Neil Kinnock, Harris had unprecedented access to the New Labour inner circle of the mid to late Nineties. As he puts it, "I was there, at the New Labour revolution." It was only later that he fell out with the Labour leadership: first over the 2001 removal of his close friend Peter Mandelson and then, two years later, over the "colossal blunder" of the Iraq war.

But unlike a raft of journalists who have turned to popular fiction with a political theme, Harris is the undisputed master of the art. This background raises The Ghost (Hutchinson, £18.99) far above the standard roman-à-clef. Harris's conversation is peppered with entertaining historical insights. "Humans have changed little over time. We think we've invented the modern world but they were making better speeches 2,000 years ago and grappling with issues of empire and terrorism." He says, almost in passing: "Cicero most reminds me of Harold Wilson. Both men knew how to keep the show on the road."

In person, Harris is a thoughtful and low-key man, who lives in rural peace and splendour in a Gothic revival house – built in 1860 by Talbot Bury, a friend of Pugin – in the Berkshire village of Kintbury. Showing me round, he stops, with obvious pride, before a tall bookshelf, holding the entire collected works of Joseph Conrad and some first editions of Greene, Orwell and Waugh. The four men – each, in their way, masters of tight, literary prose – remain models, "the ones to whom I return over and over". Propped up on a giant mantelpiece in his front hall is an invitation to the launch of the latest book by Nick Hornby, his brother-in-law and contemporary. It's hard to imagine it: two multi-million selling popular authors in one family, rather like JK Rowling and Jacqueline Wilson turning out to be sisters.

So is Lang a version of Blair? I ask Harris, as he sits, one leg informally hooked over the side of an antique chair, glass of champagne in hand. "Of course, there are parallels. An ex-PM. The backdrop, the war on terror. But I changed elements of his background, very deliberately. And it can't be Blair because Adam Lang is not interested in religion. And that is such a strong characteristic of the man."

Well, yes and no. Given Alastair Campbell's famous declaration, "We don't do God", Blair's religious side was never as apparent to the public as his marriage or his Middle East adventures. It was the one aspect of his personality that we had to divine ourselves. His intention, he insists, "is not to attack the Blairs, but to tell quite a good story. I first thought of a ghost writer years ago, in the early Nineties. I was going to do it as a three-handed play. And then I had the idea about a war crimes allegation and I mentioned it to my New York publisher, who told me to do it immediately. I came home, wrote the first paragraph on 5 January." This book has been the quickest he has written, a fact that seems slightly to worry him. "It's a bit like exams. When a thing seems easy it often isn't and it's only in the struggle, you see later, that something better has emerged."

"Jeanette Winterson said recently, 'Nobody goes to a novel for a story. They go to it for the language.' But the story is language. Story-telling has a narcotic power." Certainly, events develop at a fast pace in this book. The ghost writer, who is anonymous throughout and whose predecessor has died in worrying circumstances, starts to uncover some alarming facts about Lang and his wife. By the end, we realise that neither of them are what they seem.

But if Harris's fictional skills give The Ghost the solidity and independent reality its author hopes for, it also feels less substantive, more pallid than the historical works. This, I suspect, is partly a reflection of our modern ambivalence around politics, our craving for the impossible mix of intimacy with, and authority from, our politicians. Time has also not yet sorted the true stature of contemporary leaders.

Harris agrees. "In the new, non-tribal landscape of politics, it's hard to know where politicians are coming from, why they hold a particular view." Political parties have, he says, moved much closer to each other. "There's less hatred and violence, which is good. But I regret the collapse of oratory and debate." Blair, he says, was a "brilliant post-ideological politician" who took a wrong turning. And, I am curious to know, what does he think of Brown? So far, he is "pleasantly surprised".

So how much did he draw on his own experience of Blair and his inner circle? "Henry James always said that the formative moment in his life was a five- or six-week period over one summer. All his subsequent work drew on that period. It's the same for me." That formative time was the New Labour revolution, "when I witnessed the exhilaration and excitement first-hand".

The Ghost touches effectively on the potential inauthenticity between political master and courtier. Adam Lang has a disturbing habit of using the term "man" if he can't remember a name. In an equally cringe-worthy scene, the ghost writer interviews the ex-PM, disgusted by his own sycophancy but powerless to sidestep it. Was that scene drawn from real life? "Absolutely. That awful fawning. I've fallen into that same trap, or been aware of it."

Harris broke finally and famously with Blair – "not personally. I always got on very well with him" – over Iraq. "I don't want to be immensely moral or Pilgerish about it. But it was a bloody mistake. Yes, we need to stand up to radical Islam and defend the gains of the Enlightenment. But it morphed into an attack on Iraq. As Talleyrand said, 'It was worse than a crime, it was a blunder.' If he'd got it right, he'd still be PM."

Harris is particularly critical of modern leaders' tendency to get too close to the US – "Name me one decision over the last 15 years that has not been in the interests of America?" – and their increasing seclusion from ordinary life. "Winston Churchill walked to Parliament at the height of one of the greatest wars in human history. Leaders today are isolated by phalanxes of body guards. It's profoundly undemocratic, the way they have used terrorism as a means to protect themselves."

At a lighter level, one could read The Ghost as being about the difficulties of writing itself, something Harris knows a little about. He is, officially, John Le Carré's biographer, although the arrangement "was made before I seriously got going as a novelist and now I have my own novels to write. I interviewed half a dozen people and wrote 20,000 words, but that was 10 years ago." The project is in amicable abeyance.

So what now? After this controversial foray into contemporary life, it's back to Roman times, and the sequel to Imperium "which covers the consulship of Cicero, and the rise to power of Caesar, which should come out next autumn. After that, there will be one more volume. All three will form a single, continuous account of the last 25 years of the Roman republic."

Perhaps inevitably, we return to historical comparisons. "The main difference between politics then and now is the cruelty and barbarity. In Roman times one paid for one's mistakes with your life. But otherwise, all the plotting, the skulduggery, the theatre of it all – our sense of politicians as actors with props and gimmicks – that all remains exactly the same."

Melissa Benn's novel about the Blair years, 'One of Us', is published next March by Chatto & Windus

Biography: Robert Harris

Robert Harris was born in 1957 in Nottingham, and grew up there and in Leicestershire. He studied English at Cambridge, worked as a reporter on Panorama and Newsnight and joined The Observer as political editor in 1987. He became a political columnist for The Sunday Times and wrote non-fiction books including Selling Hitler, Gotcha!, Good and Faithful Servant, and (with Jeremy Paxman) A Higher Form of Killing. In 1992, he published his first novel, Fatherland. It was followed by Enigma, Archangel and his Roman novels Pompeii and Imperium. His fiction has now been translated into over 30 languages. His new novel, The Ghost, is published by Hutchinson. He lives in west Berkshire with his wife, Gill Hornby, and their four children.